1. Filed of the Invention
The present invention relates to optical disk apparatus that record and reproduce information to and from optical disks.
2. Description of the Related Art
Recently, techniques have been developed for making the information layer of optical disks multi-layered to increase the capacity of the optical disks. With optical disk apparatus compatible with recording/reproducing data to/from multi-layered disks, it is necessary to detect the number of information layers of the disk before the disk is determined. An example of the method of detecting the number of information layers of the disk is disclosed in JP-A-08-185636.
Technical development of optical disks as optical information media has furthermore increased the recording density of the disks compared to conventional CDs and DVDs. For example, optical disks of high-density large-capacity called Blue-ray Disks (BDs) have been developed recently. In order to increase the density of the BDs further compared to the CDs and DVDs, the BDs are designed such that the aperture number (NA) of an object lens that focuses light on the optical disk is increased and an light spot that reproduces the records is reduced in size. However, since the influence of uneven thickness of the disk substrate that protects the information layer of the optical disk on the spherical aberration is directly proportional to the fourth power of NA, a spherical aberration correction control will tremendously become difficult. Thus, it is essential to provide means for correcting the spherical aberration due to unevenness in the thickness of the disk substrate.
FIG. 1 shows one example of an optical pickup that includes spherical aberration correcting means. Reference numeral 108 denotes a laser light source. The laser light emitted by the laser light source 108 is converted from divergent light beams to substantially parallel ones by a collimating lens 107. The laser light beams then pass through a spherical aberration correction element 104 and are focused on an information surface of an optical disk 101 by an objective lens 102. Laser light beams reflected by the optical disk 101 will trace its original path and then be converted to substantially parallel light beams by the objective lens 102. The light beams are then reflected by a beam splitter 106 and focused by a second collimating lens 109 on a detector 110. The light beams focused by the detector 110 are then converted to an electric signal by which information on the disk is read out in a reproduction signal processor 115. The spherical aberration correction element 104 and the objective lens 102 are driven by actuators 103 and 105, respectively.
The detector 110 has a structure shown in FIG. 2. The detector 110 comprises four subdetectors A, B, C and D arranged as shown, as viewed in the moving direction of tracks. A center circle shown on the detector 110 schematically illustrates focusing of the laser light beams reflected by the disk. Using an arithmetic circuit of FIG. 2, a focus error signal ((b+d)−(a+c)) and a tracking error signal ((b+c)−(a+d)) are produced which are used to correct a distortion of a light spot on the disk. In this case, a, b, c and d depict values representing detected light quantities outputted from the subdetectors A, B, C and D, respectively. Focus control is performed to control the position of the disk in a light incidence direction, using the focus error signal. Positional control of the disk in a direction perpendicular to the track grooves on the disk comprises tracking control using the track error signal so as to follow the track grooves. The focus and tracking error signals drive an actuator 102 so as to correct respective follow-up error components in focus and tracking error detectors 112 and 113. Each of a reproduction signal processor 115, the focus error detector 112, the tracking error detector 113, and a spherical aberration correction circuit 114 in a servo circuit 111 are controlled by communication with a microcomputer 116. A memory has stored initial values, adjusted values, etc., in the control of the microcomputer.
As means for correcting a spherical aberration due to unevenness of the thickness of the disk substrate, a liquid-crystal spherical aberration correction element and a beam expander are used. Although the liquid crystal spherical aberration correction element is not shown, it is possible to modulate the wave front of the light beams by applying required voltages to the aberration correction element, thereby changing its refractive index. Next, the beam expander will be described in FIG. 3. In FIG. 3, a combination of positive and negative lens groups 301 and 302 is used to modulate the wave front of the light beams by adjusting the space between the positive and negative lens groups 301 and 302. In order to drive these lens groups, a combination of a piezoelectric element and a screw feed system can be used.
A method of detecting the number of information layers of a multi-layered disk will be described briefly with reference to FIGS. 4A to 4C, which illustrate focus search to be performed in an optical disk that has four information layers, for example. The focus search refers to driving a focus driver like a triangular-wave signal having a frequency, for example, of 2 Hz or so, thereby moving the optical pickup back and forth in the direction of the focus control. FIG. 4A shows information on the position of the optical pickup; FIG. 4B shows changes in the focus error signal; and FIG. 4C shows changes in the whole light quantity. In each of these figures, the horizontal axis shows time elapse. There are flex points 401, 402, 403 and 404, in FIG. 4B where the information layers pass. The focus error signal swings greatly before and after a zero point where the focus error signal passes. Thus, it is possible to detect the fact that the focus error signal has passed through the zero point using threshold detection that comprises detecting that the focus error signal has increased beyond desired reference voltages 1 and 2 provided on the positive and negative sides, respectively, of the focus error signal. The number of information layers of the disk detected at this time will be counted by the microcomputer 116.